1. Field of the Invention
The present invention relates to a method and apparatus for image processing capable of displaying information in depth direction of multiple tomographic images.
2. Description of the Related Art
Among diagnostic imaging apparatus, the ultrasonic diagnostic apparatus transmits an ultrasonic wave to a subject body, receives reverberated ultrasonic waves from positions of different sonic impedances, processes the ultrasonic reception signals to produce a tomographic image, and displays the image for the diagnosis of the subject body
The MRI apparatus utilizes the phenomenon of nuclear magnetic resonance to measure the density distribution of nuclear spin, the relaxation time distribution, etc. for the intend portion of a subject body, and produces a tomographic image of the subject body from the measured data.
The radiation CT apparatus implements the radioactive irradiation through the entire surface of the subject body, detects the radiation, and composes the detected signals to produce a frame of tomographic image.
The above-mentioned various diagnostic imaging apparatus are sometimes designed to display a three-dimensional image, and the simplest scheme for it is known to be IP (intensity projection) process.
These apparatus enables the three-dimensional observation of blood vessels, the spatial relation between the lesion and blood vessels, and the judgement of the size and nature of a tumor based on a series of projected images produced by varying the observation angle (view point) in small step, for example.
The IP process mentioned above is to produce a projected image (IP image) by extracting the largest value (or smallest value or specific value) out of pixels of all tomographic images of the subject.
In the case of the ultrasonic diagnostic apparatus, for example, which produces image data of multiple frames of images by moving the ultrasonic probe as shown in FIG. 1, a certain value of pixels is extracted along all view lines, thereby producing a frame of projected image. For extracting a certain value, the process uses a comparator as shown in FIG. 2A to compare the value of pixel in depth direction sequentially along a view line. The process of power Doppler imaging, which extracts the largest value as a projection Doppler power value, presents the relation of the Doppler power value to the depth of imaging as shown in FIG. 2B.
The IP process regarding the largest value is called MIP (maximum intensity projection) process, the IP process with regarding the smallest value is called MinIP (minimum intensity projection) process, and the IP process regarding a specific value is called specific-value IP process.
In the case of the ultrasonic imaging apparatus, for example, the MIP process regarding the largest value is effective for extracting the blood vessel wall and HV tissue such as a tumor which are displayed at a relatively high brightness, and the MinIP process regarding the smallest value is effective for extracting a tubular or hollow tissue such as a blood vessel and bile duct.
In extracting a portion such as a blood vessel based on the projection process, the MinIP process which displays a blood vessel at a low brightness is carried out. However, if there exists in another image along the same view line a pixel that is darker than the blood vessel, the darker pixel will be extracted instead of the blood vessel. On this account, the IP process involves such a problem that it is difficult to display blood vessels clearly.
Another problem is that at the extraction of a brighter portion of a blood stream in terms of a power Doppler image, particularly when the viewer intends to observe blood vessels running in the depth direction, the IP process cannot display as to which of the cross blood vessels displayed brighter is located at the front. That is, it is deficient in that positional information in depth direction is compressed and lost completely by projection.
This affair is shown in FIGS. 3A, 3B and 3C, in which the thicker-hatched portion represents the brighter portion and the thinner-hatched portion represents the darker portion in the display. With FIG. 3A and FIG. 3B showing blood vessels running at the front and rear, respectively, the above-mentioned MIP process produces a projected image as shown in FIG. 3C and fails to reveal the positional relation in depth direction of the cross blood vessels.
A similar situation is shown in FIGS. 4A, 4B and 4C, in which the thicker-hatched portion represents the brighter portion and the thinner-hatched portion represents the darker portion in the display. With FIG. 4A and FIG. 4B showing blood vessels running at the front and rear, respectively, the above-mentioned MIP process produces a projected image as shown in FIG. 4C and displays the cross blood vessels erroneously as if the brighter blood vessel in the rear is located at the front.
Similar problems also emerge in CT imaging and MR imaging. For example, a CT image of cross organs such as a bronchi and a CT image and MRI image of cross blood vessels based on contrast medium fail to reveal their spatial relation in depth direction.